mara, PR review: "the dynamic tab should be in the top bar, not a new
one below". Moves the .shell-tabs group from its own sticky row under
the header into .shell-nav itself, right after the nav indicator.
Also adds a third re-measure effect for the sliding nav indicator,
keyed on tabs.length: with tabs inline in the same flex row the
indicator measures, closing a background tab (no navigation) can
shrink the row without the hop effect's own re-measure ever firing.
Same reflow-not-navigation reasoning as the existing resize-listener
effect.
88c386c96c